Abstract A model connecting the radiation-induced noise figure shifts to the d.c. base current increases is proposed. The most significant features of the model are compared to measurements carried out on gate controlled bipolar transistors. It is established both theoretically and experimentally that ionizing radiation does not influence the noise figure behaviour when the source résistances are lower than the minimum noise figure résistance. For high source resistances, surface generation recombination centers induced by the ionizing radiation provoke noise figure increases in direct ratio to the increases of the base current. The corresponding shifts in the minimum noise figure have a small magnitude. They result from the effects of two terms, one proportional to the base current increases and the other proportional to its square root. Furthermore, the resistance source giving the minimum noise figure, decreases in inverse proportion to the base current increases. The study of noise sensitivity to internai and external transistor parameters, indicates that radiation induced noise figure increases are magnified in two cases : a) at low emitter current levels when the base surface is depleted and b) at high emitter current levels, when crowding increases carrier injection at the emitter periphery.